Provided is an assembly unit including 3×N (here, N is a natural number) number of polygonal surface parts, which extend in a vertical direction and have flat surfaces, connecting parts connecting end portions, which are adjacent to each other, of end portions of each of the polygonal surface parts, and coupling protrusions respectively protruding from the polygonal surface parts in a horizontal direction that is perpendicular to the vertical direction. The three polygonal surface parts in a clockwise direction are defined as one set, and the coupling protrusions respectively have formation positions that sequentially increase in height within the one set in the vertical direction.